Undersea mobile or autonomous systems, whether manned or unmanned, generally have no direct link to conventional positioning assets such as the global positioning system (GPS), or to other radio frequency (RF)-accessible assets. The need for navigational assistance beneath the water is further exacerbated by the general lack of available visible references. The navigation technologies available today for unmanned undersea vehicles (Mobiles) are expensive and operationally limiting. Typical systems utilize some combination of four techniques: 1) Doppler velocity logs (DVL) which provide navigational information by “pinging” the bottom of the seafloor and calculating the position of the system by “following” the system's movement with respect to the bottom. These systems, however, generally have very limited depth capability (30 meters below the vehicle is typical); 2) Long baseline (LBL), another technique which operates by relying on a series of fixed underwater transponder beacons. A transducer on the mobile system emits a signal that the beacons detect, after which the beacons emit response signals. The mobile estimates its distance from each of the beacons by timing the travel of the signals, thus enabling it to calculate its own position relative to the known positions of the beacons. This technique offers precision, but requires extensive preparation and surface expression by the deploying asset (e.g., a small craft)—a factor of importance in military applications; 3) inertial navigation, a complex and costly technique which relies on precise measurement of acceleration and rotation of the autonomous system and typically has a drift rate on the order of 1 nm/hour without compensation from a DVL or LBL; and 4) GPS surface fix, which places limitations on sea state capability and which offers precision dependent on the amount of time spent on the surface. Of these four, the LBL system is the least intrusive on, and least expensive for the vehicle, but deploying enough acoustic sources to cover a significant area (say, 5 km by 5 km) is problematic and expensive in terms of the time of the deployment vessel.
The successful and relatively low-cost navigation systems (e.g., LBL) are based in some way on the use of underwater acoustic energy transmitted from one system component to another in such a way that distance may be inferred from the time taken for the energy to reach its intended receiver. An example of such a system is described in detail in U.S. Pat. No. 6,501,704. There is a direct and well-understood relationship between the speed of sound and the range between transmit and receive components. However, the relationship is parameterized on water temperature and salinity, which are often difficult to measure. In most cases, an average sound speed is assumed, and the resulting errors are either accepted or are subject to statistical operations to reduce them.
There are two ways to use the relationship between time delay and range. In the first case, all system components are assumed to employ highly accurate and highly synchronized clocks. Thus, when a component transmits energy, it is assumed that the receiver precisely knows the time at which the transmission occurred. The actual time of arrival, therefore, is a direct measure of range, even though based on an average sound speed. The second case relies on the ability of the receiver to respond with an immediate reply to a received signal. At the location of the first transmitter, the total delay time is simply divided by two, and the range is inferred. This is a transponder system. If the intended receiver imposes a small internal delay (due to finite computation speed, for example), that has little affect, provided the original transmitter has knowledge of that delay.
LBL systems, being the most common technique currently in use, employ an assumption of sound being spherically radiated from multiple distant source nodes (using either clocks or transponder approaches). At the mobile, one employs a “simple” algorithm that relates the intersection of spheres to a common point. This point can only be calculated if the mobile has a priori knowledge of the positions of the multiple sound sources. If the positions are known in a 3-dimensional Cartesian coordinate system, then the mobile locates itself within that system. If the geo-locations of the sources are also known, then the mobile can also position itself within global coordinates. It is emphasized that the locations of the sources must be pre-programmed into the mobile prior to release of the unit.
The introduction of acoustic communications (acomms) into the positioning effort greatly simplifies the entire process. Source nodes can inform the mobile of the nodes' geopositions, which eliminates the need for pre-programming of this information. One such system, incorporating acoustic communications in combination with an LBL approach, is described in U.S. Pat. No. 5,331,602. This system, however, relies on a costly and inconvenient apparatus of multiple above water nodes.
Another technique is the Ultra Short Baseline (USBL) method, which allows for the use of a single “fixed” point of reference by having several closely positioned transducers on the remote mobile system each nearly simultaneously receiving the same signal from a fixed reference. The system calculates the phase differences of the same signal received at each of the transducers, and from these differences, is able to estimate a bearing for the signal. Present technology adopting this method is prone to many of the drawbacks of LBL in that pre-programmed information about fixed geophysical positions and depths must be coordinated prior to deployment of the system.
It is therefore an object of the invention to provide reliable and relatively inexpensive single base-node methods and systems for determining the locations of mobile underwater systems or divers without requiring pre-programming or synchronization of timing information and/or geophysical and depth information.
It is an additional object of the invention to eliminate above-water visibility of system components while determining the geophysical and depth positions of autonomous underwater systems employing acoustic modems.
Other objects of the invention will, in part, appear hereinafter and, in part, be obvious when the following detailed description is read in connection with the drawings.